This proposal is designed to test the hypotheses that: 1) there are no differences in the intake of essential fatty acids (EFAS) among groups of children with PKU fed different medical foods and normal children, and 2) there are no differences in the plasma and erythrocyte fatty acids among groups of children undergoing therapy for PKU with different medical foods and normal children. The investigators will compare EFA intake of children undergoing therapy for PKU who are ingesting Lofenalac, Phenex, Phenyl-free or XP Maximaid to intakes of their normal siblings. Plasma and erythrocyte fatty acid profiles of children undergoing therapy for PKU who are ingesting Lofenalac, Phenex, Phenyl-free, or XP Maxamaid will be compared to the fatty acid profiles of their normal siblings.